Tooth formation results from sequential and reciprocal interactions between the dental epithelium and dental mesenchyme. The molecular mechanisms underlying these interactions, however, are largely unknown. Recent studies have implicated molecules such as Msx1 and BMP4 in murine tooth development. BMP4 has been suggested to be a putative morphogen during early tooth development, while Msx1 knockout mice are toothless. The functional relationships between Msx1, BMP4 and other important molecules in tooth development are not established, however. The long- term goal of my research is to understand the molecular basis of early tooth development. The specific aims of this proposal are to clarify the genetic pathway between Msx1 and BMP4. In situ hybridization and immunohistochemistry in wild type and genetically engineered Msx1 deficient mouse molar tooth germs will be used to order these and other gene products in a genetic pathway. Recombination experiments between wild type and mutant epithelium and mesenchyme will be used to define the site of the morphologic defect. Lastly, molecular techniques, including transfection of Msx1 into appropriate cell lines with assay of BMP4 expression, and use of a binding site selection assay to identify potential Msx1 regulatory sites in the BMP4 gene, will be used to test the hypothesis that BMP4 is directly regulated by the Msx1 gene product. Other target genes for Msx1 will also be sought.